<!doctype html>

<title>04 Components - React From Zero</title>

<script src="https://unpkg.com/react@16.4.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.4.0/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/@babel/standalone/babel.min.js"></script>

<div id="app"></div>

<script type="text/babel">
// The main selling point of React is its component system
// components are used to encapsulate elements and their behaviour
// see them as a mix of controller and view of MVC

// Here we use stand alone elements and some data
var data = "world";
var reactElement = (
  <div>
    <h1>Hello</h1>
    <h2>{data}</h2>
  </div>
);

// Here the elements are encapsuled in a simple component function
// They have to start with a capital letter and return exactly ONE
// root element with or without nested elements (before React 16)
function MyComponent() {
  var data = "world";
  return (
    <div>
      <h1>Hello</h1>
      <h2>{data}</h2>
    </div>
  );
}

// Since React 16.0.0, components can return an array of elements as
// well. In doing so, no additional wrapper element is created. One
// caveat is that, similarly to what we do when rendering an array, we
// have to add a unique key to each element in the array, also don't
// forget the commas (we'll see more on this in the next lesson)
function MyComponent() {
  var data = "world";
  return [<h1 key="hello">Hello</h1>, <h2 key="data">{data}</h2>];
}

// Since React 16.2.0, we can use special "wrapper" components called
// fragments that behave the same (no extra wrapper element created)
// but remove the need to explicitly set keys (fragments do this under
// the hood) and commas.
function MyComponent() {
  var data = "world";
  return (
    <React.Fragment>
      <h1>Hello</h1>
      <h2>{data}</h2>
    </React.Fragment>
  );
}

// a component function can be used like an element
reactElement = <MyComponent />;

// this translates to a React.createElement() call
// the null indicates that no properties have been set
reactElement = React.createElement(MyComponent, null);

//  for reference a react-internal <div> tag
var anotherElement = <div />;
// gets converted to
anotherElement = React.createElement("div", null);

var renderTarget = document.getElementById("app");

ReactDOM.render(reactElement, renderTarget);
</script>